1. Field of the Invention
The present invention relates to eddy current inspection systems for detecting defects in ferromagnetic workpieces, and more particularly relates to a method and apparatus for calibrating such systems.
Eddy current inspection systems are well known for nondestructively inspecting ferromagnetic workpieces or products such as pipes, rods, tubes, etc. These systems have been particularly useful for locating anomalies, such as cracks, seams, laps, breaks and slivers in the workpiece which would not be discovered by visual inspection. The severity of anomalies varies greatly and in circumstances where an anomaly is of sufficient severity to adversely affect the function of the product when used for its intended purpose, the anomaly is, by definition, a defect.
Eddy current inspection systems generally operate by producing a varying electromagnetic field adjacent a localized area of the workpiece surface which in turn induces a localized pattern of electric eddy currents in the workpiece. The electromagnetic field is scanned along the workpiece by relatively moving the system and the workpiece. When the field is scanned across a defect in the workpiece, the defect changes the eddy current pattern. The eddy current pattern changes in turn alter the electromagnetic field and these field alterations are detected by the inspection system. The inspection system responds by marking the location of the defect in the workpiece or otherwise indicating that the defect is present. The degree of change of the eddy current pattern, and the resultant variation in the electromagnetic field, are proportional to the severity of the defect and, accordingly, eddy current inspection systems are capable of discriminating between anomalies, or defects, of differing severities.
One basic kind of eddy current inspection apparatus uses an electric coil for inducing the eddy currents in the workpiece (referred to as an "exciter" coil) and another electric coil (referred to as a "search" coil) which is positioned adjacent the location of the eddy currents in the workpiece for detecting changes in the electromagnetic field caused by changes in the eddy current pattern. An eddy current inspection system of this type is disclosed in U.S. Pat. No. 3,422,346, entitled EDDY CURRENT INSPECTION SYSTEM.
Another kind of eddy current inspection system has employed a single coil as both an exciter coil and a search coil. This type of single coil inspection system is disclosed by U.S. Pat. No. 3,688,186 issued to T. W. Judd, and entitled, METHOD AND APPARATUS FOR CURRENT FLAW DETECTION UTILIZING A DETECTOR WITH A POSITIVE AND TWO NEGATIVE FEEDBACK LOOPS (hereinafter the "JUDD patent"). In the system of the JUDD patent the coil is energized to induce eddy currents in the workpiece under test. The workpiece loads the coil to an extent determined by the nature of the workpiece and by workpiece defects which are located in the eddy currents. Changes in values of the coil loading caused by changes in the eddy current pattern produce changes in the energization of the coil and are detectable. The changes in the energization which are greater than a preset value are detected for initiating a defect signal which indicates that a defect has passed adjacent the probe coil.
During production, anomalies or slight imperfections are unavoidably produced in the workpieces. The intended usage of articles such as pipes and tubes often determines the severity of the anomalies which are allowable in the finished product. For example, when a pipe is to be subjected to high internal pressures, the existence of relatively minor anomalies in the pipe can substantially reduce the bursting strength of the pipe. On the other hand, if the pipe being inspected is not intended for use with high internal pressures, the pipe may contain relatively more severe anomalies without any adverse affects on its utility. Since eddy current inspection systems are used to inspecting different kinds of pipes and tubes, the systems must be capable of accurately detecting various degrees of severity of anomalies depending on the nature and/or the intended use of the product being inspected, so that anomalies which are sufficiently severe to constitute defects can be detected.
In the above noted inspection systems a defect signal proportional to the severity of the detected anomaly is produced. By adjusting the systems to respond to desired defect signal levels the systems are able to discriminate between degrees of severity of anomalies to indicate the presence of anomalies which are unacceptably severe, i.e., defects, for the particular kind of article being inspected. Since the difference between an acceptable anomaly and an unacceptable defect may be slight, it is essential that the systems be accurately adjustable.
In addition to the necessity for accurate adjustability of the systems, maintenance of the calibration of the systems, once adjusted, is essential. During operation the systems are occasionally exposed to changing environmental conditions, e.g., temperature changes, which tend to affect the accuracy of the systems. Similarly, wear and aging of various components tend to affect system accuracy over extended periods of use. Recalibration of the systems is therefore necessary from time to time.